1. Field of Art
This invention relates to a method for determining structural inhomogeneities, in particular due to creases or tears, in sheet material such as bank notes.
2. Summary of the Prior Art
In the testing and sorting of bank notes, for example in commercial or central banks, one generally also determines the condition thereof, e.g. the degree of soiling. The bank notes are sorted according to their condition and handled differently in accordance therewith. While bank notes in bad condition are generally withheld and possibly destroyed, bank notes in good condition can be put back in circulation.
Besides soiling one can also use, for instance, structural inhomogeneities such as creases, fine tears or small holes in the bank note for assessing the condition thereof. Structural inhomogeneities in bank notes are measured using, for instance, optical methods by which the bank note to be tested is for example irradiated with light and the reflected or transmitted light fraction measured and evaluated. However, optical methods generally have the disadvantage that measurements can be easily disturbed by ambient light and the sensitivity of the usually employed detectors and the strength of the applied light sources are generally subject to time fluctuations which likewise falsify measurement. In addition, one often requires an imaging optic comprising lenses and/or apertures resulting in a certain adjustment effort in the production and maintenance of such measuring systems.
The invention is based on the problem of providing a method for determining structural inhomogeneities in sheet material which avoids the above disadvantages.
This problem is solved by exposing the sheet material to ultrasound and the ultrasound transmitted through the sheet material and/or reflected on the sheet material is measured, thereby producing at least a first value characteristic of the transmitted and/or reflected ultrasound. From the produced first value one then determines a measure of the structural inhomogeneities in the sheet material. The use of ultrasound avoids the influence of disturbances, in particular due to ambient light. In addition, it reduces the adjustment effort.
In a preferred embodiment of the method it is provided that the transmitted and/or reflected ultrasound is measured at a plurality of places on the sheet material and for each of the places where the transmitted or reflected ultrasound is measured one produces a first value characteristic of the transmitted or reflected ultrasound. From the first values or from second values formed from the first values one finally forms a first mean as a measure of the structural inhomogeneities in the sheet material. The first mean may be the arithmetic, geometric or quadratic mean of the first or second values. One thus determines an average measure of the structural inhomogeneities in the sheet material so that relatively great local fluctuations are compensated and thus a statement can be made about the condition of the sheet xe2x80x9caltogetherxe2x80x9d with respect to creases or tears.
In another variant of the method, the first mean corresponds to the standard deviation of the first or second values from the arithmetic mean formed from the first or second values. The first mean then states the average deviation of the first or second values from their arithmetic mean and constitutes a measure of deviations of the structural inhomogeneities determined at the individual places from their average value.
In another embodiment of the method it is provided that the places where the transmitted and/or reflected ultrasound is measured are located on a track extending linearly on the sheet material. Preferably the places measured on the sheet material touch or overlap each other so as to define an interconnected track. The position and/or length and/or width of the track can be selected such that the track is not located in the area of elements additionally incorporated in or applied to the sheet material, in particular security threads, watermarks or hologram foils. This ensures that the first mean comes only from structural inhomogeneities in the sheet material itself and is not falsified by additional elements such as security threads.
Furthermore it is advantageous if the ultrasound transmitted through the sheet material and/or reflected on the sheet material is measured along a plurality of tracks in the sheet material, a first mean being formed from the first or second values for each track. From the thus obtained first means of the particular tracks one forms a second, for example arithmetic, geometric or quadratic mean as a measure of the structural inhomogeneities in the sheet material. This further averaging obtains an especially reliable statement about the structural inhomogeneities found in the sheet material on the average.
In another embodiment of the inventive method it is provided that the second values are formed from the second-order derivative of the first values. Thus causes contributions of structural inhomogeneities with greater spatial extent to be attenuated more greatly than contributions of smaller inhomogeneities. The first mean formed therefrom then takes more account of smaller structural inhomogeneities than larger ones, thereby permitting more precise statements about any existing smaller structural defects, e.g. small tears or folds.